The invention relates to a process of monitoring and ensuring the safe operation of unvented stoves, particularly of kerosene heaters, within enclosed spaces both in the normal heating condition in which the height of the flame produced by the burner is within a predetermined range, and when radiation-responsive means detect flame heights outside said range, and in operation outside the normal heating condition, with flame heights higher or lower than the predetermined range causing corresponding control signals to be generated and to be coupled to an electronic control circuit so as to cause the normal heating condition to be restored, on the one hand, and to cause a warning signal to be generated and the burner to be shut down on the other, after a corresponding timeout in case the burner consistently operates outside the predetermined flame height range for a predetermined period of time.
Further, the invention relates to apparatus for practicing the inventive process.
The European countries have recently tightened safety regulations relative to indoor air pollution caused by unvented ovens and particularly by kerosene heaters; they require such unvented ovens--such as kerosene heaters with one- or two-stage burners--to be monitored strictly for safe operation (see U.S. Pat. No. 4,390,003).
There have been known safety systems for ovens and particularly for kerosene heaters (WO 86/05860) in which, once the oven or heater has attained its normal operation, the height of the flames the burner produces may exceed a predetermined range and/or the heating means as well as the burner head and the associated piping may heat up to the point where the oven assumes an undesirable operating condition. The prior safety apparatus includes sensing means for detecting flame heights higher than a predetermined maximum and for providing a corresponding measuring and/or control signal which is coupled to actuating means responsive thereto to return the kerosene heater to the desired operating state or to shut it down. For a kerosene heater with a one-stage burner, the sensing means in the prior safety apparatus comprises two light sensors or thermal radiation detecting sensors each associated with an upper or lower limit of said predetermined range of flame heights in the normal operation of the kerosene heater. The burner's wick is re-adjusted manually or in accordance with the measuring signals generated by the light sensors as the flame height exceeds the predetermined flame height range; alternatively, the burner is shut down positively by means of a drop bar as soon as the flame height has exceeded the predetermined flame height range continuously for a pre-determined period of time.
This prior safety apparatus is based solely on a radiation-responsive detection of flame height. However, it does not satisfy the latest safety regulations as it has a number of inherent uncertainties. For example, if the burner is operated from the beginning at the lowest possible flame height, this condition will not be detected by the light sensor associated with the lower limit of the predetermined flame height range. Once the burner has burned at its lowest flame height for an extended period of time because the user has forgotten to shut down the heater, for example, there exists a great danger of the indoor space air containing inadmissible levels of CO.sub.2 since the absence of a light-responsive minimum flame height detection feature prevents the heater from being shut down automatically.
Since the wick fabric always includes irregularities, it is possible for the burner flame during normal heater operation to temporarily exceed the top limit of the predetermined flame height range--which may cause the prior safety apparatus to prematurely automatically shut down the burner although the maximum permissible CO.sub.2 level in the indoor air has not yet been reached. In an oven or heater equipped with the prior safety device, such preliminary burner shut down results in the emission of foul smell and soot, since the hot burner piping does not have enough time to cool down sufficiently to prevent the kerosene still present in the wick fabric from being burned by the heat the burner pipe will radiate, so that an intensive smell will be emitted.
In indoor spaces where an unvented kerosene heater is being operated and air ventilation is not sufficient, the CO.sub.2 will increase and the O.sub.2 concentration decrease (CH.sub.4 +2O.sub.2 .fwdarw. CO.sub.2 +2H.sub.2 O). A lack of oxygen results in incomplete combustion, however, which results in increased CO and CO.sub.2 levels. As a consequence, there exists a direct relationship between the O.sub.2 and CO.sub.2 or CO concentrations on the one hand and between the CO.sub.2 and CO concentrations on the other. As the CO.sub.2 level rises, so will the CO level.
Gas sensors of the type used in conjunction with microcomputers for automatically controlling air cleaning equipment or fans, detect via electric resistance changes, the amount of CO, H.sub.2 and a number of other organic components in gaseous environments such as kitchen vapors, cigarette smoke or automobile exhaust gases have been known per se (Figaro Eng. Inc., type TGS 800).
In medicine, it has been known to use O.sub.2 sensors for monitoring the oxygen enrichment level in artificial respiration apparatus (U.S. Pat. No. 4,495,051). Such an O.sub.2 sensor may have the form of a galvanic cell, for example, comprising a lead anode, an oxygen cathode made of gold and a weakly acidic electrolyte. A resistor and a thermistor for temperature compensation are connected between the cathode and the anode so that the galvanic cell in the form of a lead-oxygen battery discharges constantly.